1. Field of the Invention
The invention relates to a system for providing paths suitable for high frequency signals passing between an integrated circuit (IC) test equipment and pads on the surfaces of ICs to be tested.
2. Description of Related Art
Integrated circuits (ICs) are often tested while still in the form of die on a semiconductor wafer. The following U.S. patents describe exemplary probe board assemblies for providing signal paths between an integrated circuit tester and input/output (I/O), power and ground pads on the surfaces of ICs formed on a semiconductor wafer: U.S. Pat. No. 5,974,662 issued Nov. 2, 1999 to Eldridge et al, U.S. Pat. No. 6,064,213 issued May 16, 2000 to Khandros, et al and U.S. Pat. No. 6,218,910 issued Apr. 17, 2001 to Miller.
FIG. 1 is a plan view and FIG. 2 is a sectional elevation view of an exemplary prior art probe board assembly 10 for providing signal paths between an integrated circuit tester 12 and ICs 14 formed on a semiconductor wafer 16. Tester 12 implements one or more tester channels, each providing a test signal as input to one of ICs 14 or receiving and processing an IC output signal to determine whether the IC output signal is behaving as expected. Probe card assembly 10 includes a set of pogo pin connectors 26 and a set of three interconnected substrate layers including an interface board 20, an interposer 22 and a space transformer 24. Pogo pins 28 provide signal paths between tester 12 and contact pads 30 on the upper surface of interface board 20. Interface board 20 is typically a multiple layer printed circuit board including microstrip and stripline traces for conveying signals horizontally and vias for conveying signals vertically between pads 30 on its planar upper surface and a set of contact pads 32 on its planar lower surface.
Interposer 22 includes one set of spring contacts 34 mounted on its upper surface and a corresponding set of spring contacts 36 mounted on its lower surface. Each spring contact 34 contacts a separate one of the pads 32 on the lower surface of interface board 20, and each spring contact 36 contacts one of a set of pads 38 on the upper surface of space transformer 24. Vias passing through interposer 22 provide signal paths between corresponding pairs of spring contacts 34 and 36.
Space transformer 24 provides signal paths linking spring contacts 36 to a set of probes 40 arranged to contact I/O, power and ground pads 44 on the surfaces of a set of ICs 14 to be tested. A chuck 42 positions wafer 16 with probes 40 in alignment with IC pads 44 of the ICs 14 to be tested. After one group of ICs 14 have been tested, chuck 42 repositions wafer 16 so that probes 40 access the IC pads 44 of a next group of ICs to be tested.
Various types of structures can be used to implement probes 40 including, for example, wire bond and lithographic spring contacts, needle probes, and cobra probes. In some probe systems, probes 40 are implemented as spring contacts formed on the lower surface of space transformer 24 with their tips extending downward to contact IC pads 44 on the surfaces of ICs 14. Alternatively, spring contact type probes 40 are attached to the IC's pads 44 with their tips extending upward to contact pads on the lower surface of space transformer 24.
A test signal generated by a tester channel implemented within one of circuit boards 18 travels through a pogo pin 28 to one of pads 30 on the surface of interface board 20, and then travels through traces and vias within interface board 20 to one of pads 32 on its lower surface. The test signal then passes-through one of spring contacts 34, through a via within interposer 22, and through one of spring contacts 36 to one of contacts 38 on the surface of space transformer 24. Traces and vias within space transformer 24 then deliver the test signal to a probe 40 which then conveys the test signal to an IC pad 44 on the surface of one of ICs 14. An IC output signal produced at one of IC pads 44 follows a similar path in an opposite direction to reach a channel within one of circuit boards 18. As described in detail in the aforementioned U.S. Pat. No. 5,974,662, interposer 22, with its flexible spring contacts 34 and 36, provides compliant electrical connections between interface board 20 and space transformer 24. Probes 40 may be made sufficiently resilient to compensate for any variation in elevation of the IC pads 44 on the upper surfaces of ICs 14.
FIG. 2 has an expanded vertical scale to more clearly show the various components of probe board assembly 10. The horizontal area over which pogo pins 28 are actually distributed is typically many times larger than the area over which probes 40 are distributed. Probe card assembly 10 is well adapted for connecting I/O ports of tester channels that are distributed over a relatively wide horizontal area to a set of probes 40 that are aligned to access IC pads 44 that are densely packed into a relatively small horizontal area.
One problem probe board assembly 10 shares to some degree with any interconnect system, is that the signal paths it provides tend to distort and attenuate signals, particularly signals having high frequency components. What is needed is a probe board assembly for providing signal paths between an IC tester and pads on one or more ICs, wherein at least some of the IC pads transmit and receive high frequency signals.